Speckle Pattern Quality Assessment And Applications Of Digital Image Correlation

Digital image correlation method (DIC) is a new non-contact full-field optical measuring technique, which has been developed rapidly since the method was originally developed in 1980’s of last century. Because of its simple optical path, better adaptability to the environment, DIC technique exhibits many remarkable advantages than other traditional optical measurement methods. More importantly, it can be directly used in combination with various kinds of high-spatial-resolution microscopic imaging device to obtain deformation measurement under various spatial scales. DIC technique has become an important test method of modern optical mechanics and is applied to a lot of discipline domains.Through nearly 30 years of development, digital image correlation itself was perfected. But elsewhere, there are still some problems such as evaluating speckle patterns, preparing high quality speckle pattern and strain solution etc. In this paper, the evaluation of speckle patterns and the problems occurred when apply digital image correlation to specific problems are studied.First of all, this paper introduces the basic principle and basic questions of digital image correlation and systematically analyzes the key techniques which can improve DIC measurement accuracy and efficiency. These key techniques including seed point method, initial guess, initial transmission and initial update. Secondly, the measurement accuracy of digital image correlation has a close relation with speckle pattern quality. However, the artificial speckle patterns differ from each other in a thousand ways.Therefore, the reason why different speckle patterns show different results is investigated by analyzing the interpolation error and image morphology.In this paper, a simple, easy-to-calculate yet effective global parameter, called the mean intensity of the second derivative, is proposed for quality assessment of the speckle patterns. Numerical experimental studies were performed to justify the correctness and effectiveness of this new global parameter. Again, this paper by combining the mean intensity of the second derivative and the mean intensity gradient, prepared the high quality speckle patterns for the applied research as follows:(1) Thermal deformation of the WC/Cu composites were measured by digital image correlation method and the influence of WC particle size and content on WC/Cu composite expansion coefficient were obtained; (2) The local deformation field of crack tip and fracture characterization of PMMA was investigated by DGS (Digital Gradient Sensing); (3) The micro deformation field of reinforced composites was measured by combining digital image correlation method and scanning electron microscopy. The damage characteristics of the particle reinforced composite material were analyzed. The results pointed out that the distribution of particles has great influence on the damage of the particle reinforced composite materials.